The present invention relates to cross-linked or network polymers containing covalently bound chromophores with large, effective two-photon absorption cross-sections in the near-infrared spectral region and good linear transmission in the visible region.
Two-photon or multiphoton absorption occurs through the simultaneous absorption of two or more photons via virtual states in an absorbing medium, with the former being more common. For a given chromophore, these absorption processes take place at wavelengths much longer than the cut-off wavelength of its linear (one-photon) absorption. In the case of two-photon absorption (2PA), two quanta of photons may be absorbed from a single light source (degenerate 2PA) or two sources of different wavelengths (non-degenerate 2PA). Although multiphoton absorption processes have been theoretically described in 1931 and experimentally confirmed about 30 years later, this field remained dormant largely due to the lack of materials with sufficiently large two-photon sensitivity, quantified as two-photon cross-section (σ2′), which is usually expressed in the units of Göppert-Mayer (1 GM=10−50 cm4-sec/photon-molecule).
Then, in the mid-1990s, several new classes of chromophores exhibiting very large effective σ2′ values were reported. In conjunction with the increased availability of ultrafast high-intensity lasers, the renewed interest has sparked not only a flurry of activities in the preparation of novel dye molecules with enhanced σ2′ values, but also advancement of many previously conceived applications based on 2PA process in photonics and biophotonics, which are now enabled by these new chromophores. It is important to recognize the following useful features of the 2PA phenomenon based on the fact that 2PA scales nonlinearly with the squared intensity of the incident laser beam: (a) upconverted emission, whereby an incident light at lower frequency (energy) can be converted to an output light at higher frequency, for instance, near infrared (NIR) to ultraviolet (UV) upconversion; (b) deeper penetration of incident NIR light (into tissue samples, for example) than UV light that may also be hazardous with prolonged exposure; (c) highly localized excitation as compared with one-photon processes, allowing for precise spatial control of in situ photochemical or photophysical events in the absorbing medium, thereby minimizing undesirable activities such as photodegradation or photobleaching; and (d) fluorescence, when properly manipulated, that would allow for information/signal feedback or amplification in conjunction with other possible, built-in effects such as surface plasmonic enhancement.
It is anticipated that further ingenious utilization of these basic characteristics will lead to practical applications other than the ones that have already emerged in such diverse areas as biomedical fluorescence imaging, data storage, protection against accidental laser damage, microfabrication of microelectromechanical systems (MEMS), photodynamic therapy, etc. In the past decade or so, significant advances have been made in the fundamental understanding of general structure-property relationships that have led to the design and synthesis of two-photon absorbers with very large cross-section values. Although further enhancement of 2PA cross-section is still possible as suggested by a number of theoretical studies, for certain applications, the two-photon-property requirement has essentially been met by the state-of-art chromophores. Because of the possible property-processing/fabrication tradeoff, the secondary properties, e.g. thermal and mechanical properties, that are important to material processing into various useful forms (films, coatings, fibers, windows etc.) and configurations should be addressed. For the aforementioned solid forms, polymers may offer many advantages such as the flexibility in fine-tuning the material properties and the availability of many processing options.
Polyurethanes are one of the most versatile commodity polymers that are found in general applications such as coatings, adhesives, composite matrices, shape memory polymers, etc. They are best known to the general public in the form of flexible foams that can be found in upholstery, mattresses, earplugs, and packaging, as well as rigid foams in the insulation for buildings, water heaters, refrigerated transport, and commercial and residential storage refrigeration. In more advanced applications, polyurethanes have been shown to be promising as matrix polymers for electro-optical devices (see, e.g., D. S. Won et al., Polymer International, 2010; 59:162-168) and two-photon lithography (see, e.g., A. Ambrosio et al., Applied Physics Letters, 2009; 94, 011115).
Polyurethanes are typically formed under anhydrous conditions by reacting a polyol (an alcohol with more than two reactive hydroxyl groups per molecule, A(OH)x, where x=2,3,4,5,6.) with a diisocyanate or a polymeric isocyanate (B(NCO)y, where y=2,3,4,5,6.) under the influence of heat and/or in the presence of suitable catalysts and additives. Linear polyurethanes are typically produced from the generic reaction of A(OH)2 and B(NCO)2. Cross-linked or network polyurethanes are generated from the various combinations of A(OH)x+B(NCO)y, where either x or y or both are greater than 2. For advanced optical applications where two-photon properties are required, a practical advantage of using polyurethane as a matrix polymer is that a good selection of the polyol and polyisocyanate monomers, which may be cured to form glass-like clear films, lenses, and windows, are readily available in large quantities and at low cost. An attractive feature of the cross-linked polyurethanes is the possibility of tailoring the processing and fabrication conditions to be solventless and to be compliant with green manufacturing practices.
Accordingly, it is an object of the present invention to provide two-photon active thermosetting polymers that: (i) are generated from thermally reactive, multifunctional monomers under practically solventless conditions; (ii) are comprised of network structure at the molecular level, leading to amorphous and visually transparent solids; and (iii) contain in their repeat units the essential components such as electron-donating triarylamine and electron-accepting 1,3,5-triazine moieties, as well as conjugated bridges such as 2,7-fluorenyl, para-phenylene, and related para-phenylenevinylene, when in combination, are known for their high 2PA response.
A specific object is to provide 2PA solid materials prepared from solventless thermal polymerization of a novel polyhydroxylated AFX chromophores with commercially available organic bis(isocyanato) and tris(isocyanato) monomers resulting in 2PA-active, cross-linked polyurethane boules containing chromophores with the structural motif in which a 1,3,5-triazine core is triply connected to tertiary amino endgroups via 9,9-dialkyfluorenyl bridges.
Other objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.